1. Field of the Invention
The present invention relates to a vertical lathe useful for ultra-precision machining of, for example, a mold for molding a Fresnel lens, and more particularly to a vertical lathe suited for machining of lens grooves having gradually varying inclination angles, such as the lens grooves of a Fresnel lens.
2. Background Art
FIG. 9 shows a Fresnel lens which is utilized as a condenser lens in a precision optical instrument. The Fresnel lens 2 has concentric or spiral lens grooves 4 each having a V-shaped cross-section. As shown in FIG. 10, one side surface of each lens groove 4 is called Fresnel surface f, while the other side surface is called rise surface r. The Fresnel lens 2 is characterized in that the inclination angle (Fresnel lens angle Ø) of Fresnel surface f slightly differs between adjacent grooves and gradually varies.
Fresnel lenses have recently been produced by injection molding processes. In machining of a mold for molding a Fresnel lens, it is necessary to machine with precision lens grooves whose inclination angles of Fresnel surfaces f vary gradually.
Vertical lathes have been employed as a machine tool for carrying out machining of such gradually varying grooves (see Japanese Laid-Open Publication No. 1998-142405).
Conventional vertical lathes for carrying out machining of such gradually varying grooves include a tool post swivel mechanism for changing the nose angle of a cutting tool. FIGS. 12 and 13 show tool post swivel mechanisms of conventional vertical lathes. In vertical lathes that employ the respective tool post swivel mechanism, a saddle moves linearly and horizontally in the X-axis direction and a ram moves linearly and vertically in the Z-axis direction by means of a mechanism common to the lathes. For swiveling of the respective tool posts in the B-axis direction, however, the lathes employ different mechanisms.
Referring first to the tool post swivel mechanism of FIG. 12, the tool post 110 is of a type provided with four cutting tools 100. A swivel head 112, which supports a swivel shaft 114 by an air bearing, is mounted to the lower end portion of a ram 111. The swivel shaft 114 is secured in the center of the tool post 110, and the four cutting tools 100 are held symmetrically about the swivel shaft 114 at an angle of 90° with each other. In FIG. 12, reference numeral 115 denotes a servo motor for driving the swivel shaft 114.
In the tool post swivel mechanism shown in FIG. 13, on the other hand, an R guide 118 having an arc-shaped guide surface is provided at the lower end of a ram 117. A swivel base 119 swings or tilts by the guide of the R guide 118 so that a cutting tool 100, which is held by a tool post 120 fixed on the swivel base 119, is allowed to swivel with its nose as the center of swiveling. Thus, only one cutting tool 100 is held by the tool post 120. As a drive mechanism for the B-axis swiveling is employed a mechanism (not shown) which converts a linear movement, for example by means of a ball screw mechanism, into the tilting movement of the swivel base 20. The tool post swivel mechanism of FIG. 13 differs in this respect from the tool post swivel mechanism of FIG. 12 which simply swivels the tool post 110 by means of the B-axis swivel shaft 114.
Machining (cutting) of lens grooves in a mold for molding of a Fresnel lens, using the conventional vertical lathe having the tool post swivel mechanism shown in FIG. 12, is carried out in the following manner. First, while a cutting edge 101 of the cutting tool 100 is inclined in conformity with a Fresnel lens angle Ø, as shown in FIG. 11, the cutting tool 100 is lowered while a turning table is rotated so that by synthesized feed of the tool by X-axis and Z-axis movements, cutting of a rise surface r is effected to a certain cut-in depth in the rise surface r of a workpiece W. When the cutting tool 100 has reached a predetermined depth (in the Z-axis direction), the cutting edge 101 of the cutting tool 100 is transferred to the workpiece W to create a Fresnel surface f. The rise surface r cannot be cut into a mirror surface by the above cutting operation because of the synthetic feed by X-axis and Z-axis movements.
Next, following the machining of the Fresnel surface f, finish machining of the rise surface r with a cutting edge 102 is carried out by the following operation.
The cutting tool 100 is released from the workpiece W, and the tool post 110 is swiveled so that the cutting edge 102 of the cutting tool 100 matches the inclination of the rise surface r. By the swiveling of the tool post 110, the X-axis position of the cutting tip of the cutting tool 100 is displaced widely from the lowermost point at the bottom of the lens groove. Accordingly, the cutting tool 100 must be moved by X-axis movement to align the cutting tip of the cutting tool 100 with the lowermost point at the bottom of the lens groove. Thereafter, the cutting tool 100 is lowered by Z-axis movement to carry out machining of the rise surface r to a certain cut-in depth.
The angular change of the cutting tool 100 thus involves X-axis movement and Z-axis movement, which may entail errors, such as a positioning error and a displacement of the position of the cutting edge due to an error in the X-axis linear feed motion. Such errors lead to a deviation of the position of the nose of the cutting tool 100 from the intended lowermost point at the bottom of each lens groove, resulting in errors in the pitch and the depth of lens grooves.
In the case of the tool post swivel mechanism shown in FIG. 13, on the other hand, the cutting tool 100 swivels with its nose as the center of swiveling, as described above. It is, therefore, theoretically possible that after machining of a Fresnel surface f, the cutting edge 102 of the cutting tool 100 can be matched to a rise surface r only by swiveling the swivel base 119 while keeping the nose of the cutting tool 100 at the lowermost point at the bottom of the lens groove.
In actual machining, however, it is very difficult to position the nose of the cutting tool 100 at the center of swiveling and keep the position during the swiveling operation. It is noted in this regard that the accuracy of movement (swiveling) is greatly influenced by the configurational accuracy (roundness) of the guide surface of the R guide 118 which guides the swivel base 119, and it is very difficult to provide the guide surface of the R guide 118 with high roundness. Thus, depending upon how the swivel base 119 makes contact with the guide surface of the R guide 118, it is possible that the position of the nose of the cutting tool 100 deviates from the intended position. Further, according to the tool post swivel mechanism shown in FIG. 13, because of the large radius of swiveling of the cutting tool 100, the swivel mechanism of a considerable weight is disposed on the front side of a cross rail. This tends to cause a torsion in the cross rail, which may significantly affect the position of the cutting tip of the cutting tool 100, leading to displacement of the actual position of the cutting tip from the intended position.